Insulated heater and method of manufacture



June 23, 1942. E. R. WAGENHALS ETAL 2,287,460

INSUMTED HEATER AND METHQD OF MANUFACTURE Filed Nov. 29, 1940 MA CHINE FOR WIND/N6 WIRE 0N MANDREL avs/v/ [NV EN TORS EDWARD R. WA GENHA L8 AN NCERSHARDLOW ATTORNEY.

Patented June 23, 1942 2,287,460

UNITED STATES PATENT oFFIcE INSULATED HEATER. AND METHOD OF MANUFACTURE Edward R. Wagenhals, Summit, and Lawrence R.

Shardlow, North Arlington, N. J., assignors to Radio Corporation of America, a corporation of Delaware 1 Application November 29, 1940, Serial No. 367,774

8 Claims. (01. 20147) Our invention relates to insulated high resistance heaters and methods of manufacture for use in electron discharge tubes or analogous devices.

Such heaters must be made either from short lengths of very small diameter wire or from longer lengths of relatively larger wire. The use of extremely fine wire is disadvantageous in that the drawing and handling of such fine wire is extremely difficult and when made into heaters the temperature during operation may be so high as to cause rapid vaporization of the heater wire and cause excessive electrical leakage between the heater and other elements of the device. The use of longer lengths of relatively larger diameter wire may be disadvantageous due to the small volume in which the heater must be inclosed. The usual practice is to form a folded heater of continuous insulated wire or to form a single helical coil, coiled coil or reversely wound double helical coil which is insulated by spraying or dipping in insulating material. Various methods have been proposed for making such heaters which includes a step of helically coiling a fine wire about a mandrel which is subsequently dissolved. The heater wire is then insulated such as by spraying a suspension of aluminum oxide or Alundum in a nitro-cellulose binder to completely cover the coiled wire. While this method is suitable for insulating such heaters when formed of relatively large diameter wire, it is very difilcult to use this method for relatively fine wire unless an insulating core rod is used to support the wire. The use of a core rod inherently increases the mass of insulation and consequently lengthens I the time necessary for the heater to reach operating temperature.

It is an object of our invention to produce an insulated coiled heater from fine wire, the heater being self-supporting during the manufacturing process. Further objects are to provide a fine wire heater and method of manufacture characterized by short heating time and long life, to simplify the manufacturing technique of making insulated heaters, and to provide a coiled heater which is less susceptible to failure by burn-out adjacent the ends thereof.

In accordance with our invention the dimculties of manufacturing such heaters are avoided by applying the insulating coating by a method new to the insulating of coiled or coiled coil heaters. Therefore, more particularly in accordance with our invention, we have constructed heaters using a single helix by winding the heater wire helically about a mandrel and applying the insulation by any of the usual methods before removal of the mandrel. Likewise the single helical coil may be used to form a coiled coil or reversely wound double helical coil whereupon the insulation is applied prior to the removal of the mandrel on which the first helical coil is formed.

These and other objects, features, and advantages of our invention will become apparent upon consideration of the following description and the accompanying drawing in which:

Figure 1 is a diagrammatic view of a coil-winding machine and associated apparatus and Figure 1A is a view of certain optional associated apparatus for making heater coils in accordance with our invention,

Figure 2 is a greatly enlarged perspective view, with a portion thereof sectionalized, of an insulated heater following an intermediate step of our manufacturing process,

Figure 3 is a view of a completed heater of the coiled-reversed double helical coiled type, and

Figure 4.- is a cross-section of a portion of the heater shown in Figure 3 taken along the lines 4-4.

Our invention may be practiced by utilizing any standard coil-winding mechanism A for winding 2. filament wire on a mandrel and while our invention will be described particularly with reference to a heater of the single helix type it is nevertheless directly applicable to heaters of any form wherein the primary coil winding is helically wound about a soluble mandrel. Referring to Figure l the mechanism A may include a spool ill for the mandrel wire it which is usually molybdenum wire of relatively large diameter. This mandrel wire is drawn through a winding head it driven by a pulley 83 by means of a belt l4 leading to adriving motor not shown. The coil winding head includes a spool l5 for the heater wire 16 which is usually of tungsten. This filament wire is led over pulleys l1 and I8 and as the head l2 rotates the mandrel wire is drawn from the spool l0 and the filament wire is wound around the mandrel wire to produce a mandrel wound coil I9. The mandrel wound coil l9 may be wound on a reel although we have shown a pulley 2| over which the mandrel wound coil I9 passes, whereupon it may be drawn through a heat treating oven or furnace 22 to clean and set the tungsten wire so that when out into coil lengths 23, such as by severing mechanism 24, it retains its single helix form. While we have shown in Figure 1 the mandrel wound coil is passing directly into the heat treating furnace 22, the step of heat treating may be performed. at a later time as hereinafter explained. Follow ing the severing operation the lengths 23 while retained on the mandrel may be used as primary coils in formingdouble helical coils on a second mandrel or the mandrel coil l9 may be formed directly into a coiled coil such as by the method disclosed by Wadsten, 2,136,649, the final product being a heater in which the mandrel on which the primary coil was formed is retained.

In accordance with our invention we have found that notwithstanding-the presence of an insulating coating covering the coiled heater wire and contrary to expectation it is still nevertheless possible to remove the mandrel without injury to the insulating coating or injury to the heater wire. In this manner we'are able to obtain a heater which is self-supporting during the manufacturing process and especially during the steps of applying and sintering the insulating material applied to the heater.

Therefore in accordance with our invention and when making single coiled insulated heaters the single coil lengths 23 may be supported by their ends and sprayed with an insulating material such as aluminum oxide or finely divided Alundum prior to the removal of the mandrel. When coated prior to the removal of the mandrel the single helix wound thereon can be handled as a single wire which is self-supporting during the coating process.

If it is desired to cover the mandrel wound coil IS with insulation continuously, the arrangement of Figure 1A may be utilized. In Figure 1A the mandrel wound coil l9 after passing over the pulleys 2| and 30 is drawn through a spray 3| of insulation such as from the spray gun 32 whereupon the insulated coil containing the mandrel is drawn through a furnace 33 which sinters the deposited insulation and volatilizes any binders or liquid vehicle used in the spraying operation. The coating of insulation may be applied by a drag process'but in any event the insulation is applied prior to removal of the mandrel. Obviously the insulating material will be deposited only on the surface of the heater wire not in contact with the mandrel since the inside of the coil is occupied by the mandrel. In the arrangement of Figure 1A it is unnecessary first to set the wire on the mandrel by heat treatment because the furnace 33 both sets the wire and sinters the insulating material at the same time.

The Alundum or aluminum oxide insulation is preferably of small particle size and we have found commercial Alundum grade 38-900 particularly suitable when mixed with a binder comprising 2790 cc. butyl acetate 950 cc. diethyl oxalate and 86 grams of 60 to 80 second nitrocellulose containing approximately 30% alcohol to 1875 grams of the Alundum, the suspended Alundum being sprayed on the coiled wire prior to the removal of the mandrel.

Particularly good results have been obtained in following our method when using an insulating material which is porous or fluffy. Thus, if the material is applied by spraying, a relatively dry spray is applied to the heater wire. Such a dry' spray produces a more porous coating than that produced by a drag process and decreases the time required to remove the mandrel.

The sintering of the insulation is preferably lowing proportions by volume:

done in a hydrogen atmosphere, and to prevent embrittlement of the tungsten wire the hydrogen is bubbled through water prior to its introduction Per cent H2O 20 Concentrated H2804 30 Concentrated HNO: 50

After the mandrel wire was removed the coils were washed in boiling distilled water until all traces of acid and molybdenum salts had been removed whereupon the heaters were ready for use.

electron discharge devices have been made by our process, it being found that in some cases it was difficult to dissolve the molybdenum mandrel after the firing of the insulation. Apparently some reaction occurs during the firing step and we have found it desirable although not always necessary to immerse the insulated coils in ammonium hydroxide to remove any reaction products inadvertently formed during the firing process. Therefore prior to immersing the coils in the nitric acid dissolving solution, they may be immersed in concentrated ammonium hydroxide for a period of approximately one-half hour followed by boiling in distilled water for about five minutes. The insulated coils are then immersed in the nitric acid-sulphuric acid solution mentioned above followed by washing in boiling distilled water to remove the reaction products of the molybdenum wire and acids.

Heaters made in accordance with our invention are characterized by rapid heating time since a very small volume of insulation may be applied.

Referring to Figure 2 the mandrel wire ll forms a core supporting the heater wire IS on which the insulating material 34 is deposited prior to removal of the mandrel. The mandrel prevents the insulating material from flowing within the coil and prevents the interior of the coil from being filled with insulating material. Thus no insulation covering whatsoever is provided over the heater wire in contact with the mandrel and the interior of the coil is hollow, there being no insulation in the space originally occupied by the mandrel. In use our heaters have been found to operate at a higher temperature than heaters covered by agreater amount of insulation and in some cases burn-outs have occurred adjacent the section of the heater welded or pinched to the current carrying lead.

Therefore in accordance with a further teaching of our invention and referring to Figures 3 and 4 a heater which is coated with insulation except at the ends thereof and from which the mandrel has been removed is provided with current carrying tabs 35 which are welded or otherwise fastened to the ends of the heater. Alternatively the ends of the heater may be welded or fastened to the current carrying leads directly without any intermediate conductor. During the welding or fastening of the ends of the heater to the leads or tabs 35 some of the insulating material 34 is chipped from the heater and localized heating may occur. We have found that if after welding or fastening the heater to the leads or tab 35 a mass of insulation is applied to the ends Several million insulated heaters for use in J of the heater. such burn-outs are avoided. More particularly the coil of the heater adjacent the lead or tab 35 may be filled with insulation so that the mass of this insulation per unit length thereof along the heater approaches or is greater than the mass of insulation per unit length over the insulated portion of the heater. Referring to Figure 4 the mass of insulation 36 is applied after the heater wire I I has been welded or fastened to the tab 35. A mixture of 30 grams of the above grade Aundum with 7 cc. S brand sodium silicate as supplied by the Philadelphia Quartz Co. with 7 cc. water is applied by a finely pointed brush and the-material flows into the interior of the coil by capillary attraction. This mixture need not be fired as in the case of the heater coil insulating material but may be air dried prior'to in an evacuated envelope or the heat developed by the heater during first operation is sufficient to liberate the volatile components and to set the mass of insulation.

While we have indicated the preferred embodiments of our invention of which we are now aware and have also indicated only one specific application for which our invention may be employed, will be apparent that our invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure and method oi manufacture used and the purpose for which structure is employed without departing from the scope'oi our invention as set forth in the appended claims.

3E claim:

l. Method of making a coiled heater which comprises helically winding a wire abouts, molyb- *nun'i mandrel, coating the wire with an eleccally insulating material, firing the wire and ing to sinter the coating, immersing the wire ammonium hydroxide to dissolve rough said coating any molybdenum oxide in- 'itly had during xllring and subse sing the coated wire in an acid wire and said coating to dissolve making a coiled electrically inheater from a tungsten wire which comprises first helically coiling the wire about a mandrel, heating the wire and mandrel to set the wire, applying a coating of electrically insulating material to the coiled wire, firing the coated wire a hydrogen atmosphere to which water vapor has been added to prevent embrittlement of the tungsten wire to sinter the coating, immersing the coated wire and mandrel in a solution ln' which molybdenum oxides are soluble, and then dissolving the mandrel through said coating while maintaining said coating and said wire intact.

3. Method of making acoiled heater which comprises the ste s of winding a fine tungsten wire about a molybdenum mandrel, to form a coil onsaid mandrel, winding said coil and mandrel about a second mandrel to form a coiled coil, removing said second mandrel, covering said coiled coil with a coating of electrical insulation,

firing said coated coiled coil to sinter said insulation, immersing said coiled coil following the application of said coating of electrical insulation in a solution to dissolve through said coating any molybdenum oxide incidentally formed during said firing step and dissolving said molybdenum mandrel through said coating without injury to said insulation.

4. Method of making a coiled heater comprising the steps of helically coiling a fine wire about a long soluble mandrel, heating the coiled wire to set the wire, cutting the coil and mandrel to form a plurality of relatively short coil sections, coating the coil with the exception of a small length adjacent the ends with a coating of insulation, firing the coiled wire, coating and mandrel to sinter the said coating to form a substantially solid mass including the wire, coating, and mandrel, dissolving the mandrel from the coiled wire leaving the fine wire bare on the inside of said coil and on the ends thereof, welding said coil at each end to a current carrying terminal, and applying insulating material to the bare ends of said coil to fill the ends of the coil with said material. I

5. An insulated heater comprising a helical coil of fine wire, a coating of insulation covering only the exterior surface of said coil leaving the interior surface uncoated and bare, and a mass of insulation within and filling the ends of said coil which mass is greater per unit length adjacent the ends than the mass of insulation per unit length over the principal length of said coil.

6. The method of making an electrically insulated coiled heater which comprises helically coiling a wire about and in supporting contact with a metal mandrel, forming a porous coating of electrically insulating material over the entire lateral surface of said wire and said mandrel not in mutual contact, ng said wire and coating to sinter said coa .g without destroying the porous character of said coating, and then dissolving said metal mandrel through said porous sintered coating without injury to said porous sintered coating by immersing the coated wire and mandrel in an acid solution inert with respect to said wire and said coating.

7. Method of making a lied heater comprising the steps of helically g a fine wire about and in supporting h a long soluble mandrel, heating mandrel to set the wire, out. 1 i i Y andrel to form a plurality relatively sections each supported by a mandrel sec coating the coil section with the exception of a small length adjacent the ends with a coating of insulation, firing the coated coil sections to sinter said coatlng without destroying the porous character of the coating to form a substantially solid porous mass including the wire, coating, and mandrel, and dissolving the mandrel from the coated coil sections leaving the fine wire "care on the inside of said coil over the areas originally in supporting contactwith said mandrel and on the ends thereof.

8. The method of making an electrically insulated coiled heater which comprises helically coiling a wire about and in supporting contact with a metal mandrel, forming a porous coating of electrically insulating material over the entire lateral surface of said wire and said mandrel not in mutual contact, firing said wire and coating to sinter said coating without destroying the porous character of said coating, immersing. 

